The present invention is directed to a method for forming and/or dewatering a web such as a fibrous web in a paper or cardboard machine, in which web formation is accomplished with a forming fabric and/or a particular band serving as a machine element for transporting the web forwardly and supporting the same. The present invention is also directed to an apparatus for forming and/or dewatering the web, e.g. a felted web, within a paper or cardboard machine, which comprises a band-like member guided by rotatably supported rolls, and serving as a machine element for both supporting the web itself and transporting the same forwardly. The present invention is also particularly directed to a processing band that is utilized for forming and/or dewatering such a fibrous web.
Conventionally, a porous paper web running through a paper machine is dried initially by dewatering on a fabric, such as a wire, or between two fabrics. Such initial dewatering reduces the moisture content of the paper web to a value u.sub.v =5.7 to 2.3 (g of H.sub.2 O per g of dry matter), depending upon the brand of paper. Subsequently, further removal of water from the web is accomplished in the press section of the paper machine by passing the web in the nips of press rolls in which a porous felt is generally also applied to enhance the dewatering. The moisture content of the paper web is generally reduced in the press section of the paper machine to a value u.sub.v =1.6 to 1.2. Following the press section, the paper web is dried through evaporation, e.g., utilizing multiple cylinder dryers, where the web to be dried is placed in contact with steamheated, smooth-surfaced drying cylinders. The ultimate moisture content of the paper web is generally in the range u.sub.v =0.05 to 0.1.
As conventionally known, the web is dewatered by suctioning with the aid of stationary upper and lower suction boxes or rotary suction rolls. Suctioning effect is also produced with so-called foil lists. Furthermore, both gravitational and centrifugal forces are utilized in the dewatering process.
In the press section of a paper machine, water is expressed from the web in a conventional manner, within a nip defined by two rotating rolls, or within a nip defined by a roll and a shoe urged against it. The fibrous web, e.g. a paper or cardboard web, is then interposed either between a fabric (a so-called "felt") and a press roll which is often formed from granite, or the fibrous web is disposed between two such felts. The press nip itself may also be defined by a granite roll and a cooperating roll with a smooth or recessed surface, this latter cooperating roll also being a so-called suction roll. Conventionally, a sandwich-type structure supporting the web passes through the nip, such structure comprising various bands, coatings, felts, along with the running fibrous web while the nip itself may be constituted by a plurality of rolls rotating on both sides of the sandwich-type structure passing therethrough, or only one roll disposed to rotate on only one side of the sandwich-type structure thereof (e.g. when a shoe forms the other press component on the opposite side of the sandwich-type structure). Thus, such a press nip may be provided and disposed in a variety of ways, as conventionally known. As noted above, one of the rolls defining the press nip may be conventionally replaced with a stationary press shoe, in which case a deformable band is disposed between the shoe and the felt run, while lubricant is also introduced between the shoe and the band itself.
Although pressing at elevated temperatures within the nip has been carried out in the dewatering/pressing operation in order to alter the viscosity of the water and the elastic characteristics of the fibrous web itself, thereby attaining a higher dry matter content of the running web, it is not possible with conventional pressing procedures to attain the ultimately-desired dry matter content of the running web for intended use. The remainder of the water contained within the web has to be removed therefrom by evaporation along the surface of rotating drying cylinders. In this drying step, various drying wires and fabrics have been utilized, as well as particular air streams designed to remove escaping water vapor. It is well known that drying cylinders may be heated in a number of different ways, the most common of which is to conduct hot steam directly into the cylinders. The cylinders and fabric also serve as a mechanism for transporting the running web itself. The web may also be dried, transported, and supported by means of an air cushion.
In addition to drying of the web, correction of the dry matter profile is also carried out. In other words, water may be added to the running web in a controlled manner, so that a desired dry matter profile is obtained across the web. Thus as conventionally done, the moisture content and profile of the running web are controlled so that quality requirements imposed on the final product, e.g. paper, will be attained as best as possible. However, drying of a paper web by evaporation is not energy efficient. Drying by evaporation consumes remarkable quantities of energy, since the energy required for evaporation of water is about 2,500 kJ/kg.
In conventionally-known paper manufacturing techniques, water drains, for example, into save-alls disposed along the machine, into suction boxes, into holes of suction rolls, or into cavities along a recessed-surface of such rolls, and into the felts themselves, at the very same location where the water is removed from the web. An alternative to this type of water removal is to utilize a felt which receives and conveys water while exiting from the press nip. However, this type of operation is clearly disadvantageous, since water remaining in the felt may re-wet the web if the web and felt are in contact with one another after exiting from the press nip. However, other technical solutions have not been conceived.
U.S. Pat. No. 4,357,758, issued Nov. 9, 1982, discloses a procedure for drying a porous web-like object such as paper, or a granular material such as peat, or a solid material, such as wood. In the particular procedure disclosed therein, the object to be dried is placed in contiguity with a fine-porous suction surface saturated with a liquid and which is in liquid communication with a volume of liquid which is maintained at an underpressure or reduced pressure relative to the pressure of the liquid in the object to be dried. In this patent, an apparatus for carrying out this method is disclosed, which comprises a finely-porous, liquid suction surface with radii of pores therein within the range of about 0.05 to 2 microns. The suction surface is saturated with liquid by placing the same in communication with liquid confined in a liquid volume defining means which itself communicates with means for creating an underpressure or vacuum.
Additionally, drying means for a fibrous web is disclosed in the above-mentioned patent, which comprises at least one water-suction cylinder, in conjunction with which a permeable fabric or band is disposed so that the web is urged against the finely-porous, water-suction surface. This particular type of apparatus offers great potential possibilities for water-suction drying, in principle. However, there is a practical problem encountered in that it has been difficult at contemporary high paper machine speeds (e.g. about 17 m/s) to realize sufficiently long delay times of the running web in conjunction with the finely-porous, water-suction surface of the cylinder which a water-suction drying process requires, without having to use cylinder diameters which are undesirable from the viewpoint of practical embodiments, or extremely numerous cylinders. Additionally, in this particular type of apparatus, it has been necessary to utilize disadvantageous structural designs regarding pump means with which the liquid space communicating with the water-suction surface is placed under the underpressure or vacuum for water-suction drying.